As a prefix, uni-, a term to denote singularity.

universe, unique, unisexual, unitard

Uni is sea urchin roe from the female side of the organism. It is considered a delicacy to sushi lovers around the world. The best way to describe uni to western people is that it is like eating something that tastes the way a natural sea sponge smells; with the added benefit that uni also has the consistancy of cream of wheat.

Uni's taste varies with the season. Over the course of a year, its flavor can range from being slightly sweet to midly bitter.

Exceuse me. But just to clarify.

Uni is sea urchin roe not sea urchin, whether used as sushi or not.

When it is served as sushi, it is generally presented as a maki, but not rolled inside the rice. Instead, a loose collar of nori is folded around sushi-rice with a space above the rice which is filled with the roe.

The supreme goddess of the Etruscan pantheon. She is the goddess of the cosmos, and the city goddess of Perugia. Together with her husband Tinia and the goddess Menrva she forms a triad. Her son is the hero Hercle (clearly Hercules / Heracles). Uni is identical to the Greek Hera and the Roman Juno.

See also: Etruscan Mythology

Uni - n., Japanese

Writing
It has three different spellings in kanji. One form means sea chestnut, which comes from its appearance. It is also often spelled out in hiragana. Japanese scientists like to use katakana to spell living things in general.

Definition¹
(1) sea urchin, or (2) salted sea urchin roe (meaning the eggs) that is used for food. (Sorry to disagree, sensei.) There are many kinds of uni, of various colors and sizes.

As a sushi item
Best eaten salted, uncooked, and still juicy. Uni is indeed a delicacy, and as it is often the case with the more popular sushi items, it is higher in calories and higher in price. Uni is served either as neri uni (egg paste) or tsubu uni (with the eggs still having form), and it is in season during Spring.

A common misunderstanding
A common misunderstanding held among Americans, told to gross out other Americans, is that uni is the semen part. Actually, only the eggs of sea urchins are food. They are most likely confusing uni with shirako, which comes from a different fish.

Super Mario Brothers 3
Appears in some SMB3 sea levels as a big purple square shaped character officially known as unitaro.

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Source(s)
1 - http://dictionary.goo.ne.jp/ (Page in Japanese. Uni also means charcoal, but people usually don't use that word.)


2009-02-25: Update and a correction: According to a TV show on uni, apparently it's both the eggs and the semen that are eaten in Japanese cuisine. You either get egg packs or semen packs from a single sea urchin. Surprisingly, they both look and taste very similar, and are sometimes not distinguished at food markets.

Uni (pronounced "Yoo-ni") is a main character in the 1983 Marvel Productions Ltd., D&D Enterprises and TSR nonlinear animated series, Dungeons & Dragons. Uni is voice acted by Frank Welker.

Uni is a female baby white unicorn. It makes an appearance in the series in the opening sequence, where it is being chased by Tiamat, who must have been going through hard times to put so much effort into trying to catch such a small animal. When six children myseriously drop from the sky and change into a number of Dungeons and Dragons character classes, Uni is rescued and subsequenty adopted by one of the children, Bobby, as a pet.

Uni speaks in a peculiar cute sort of way, squeeking out syllables and generally repeating the last thing said if she thought it was intelligent. However, Uni's apparent cuteness and clumsiness belies a powerful intellect. In the episode In Search of the Dungeon Master it carries out a complicated plan to distract orcs, thereby enabling Sheila to recover the groups' weapons. In P-R-E-S-T-O Spells Disaster Uni even manages to cast a spell from Presto's hat, saving him from a rampaging dragon.

In the episode Valley of the Unicorns, Uni finds other members of its own kind. While it briefly stays to socialize, it eventually makes up its mind to rejoin the group on their travels. It is in this episode that Uni learns it can teleport, a skill that is used to a small extent later in the series.

To a certain extent it would be true to say Uni's clumsiness holds the group up. In The Dragon's Graveyard, Uni trips over its own hooves and gets blasted by Venger. Sheila implies that there is no hope and Uni will die, but a magic net in the Dragon's Graveyard heals the unicorn of its wounds enabling them to continue. Uni's presence often causes Bobby to hesitate vital seconds before going through a portal home, and this often means that by the time he is willing to go through the portal isn't there anymore.

Uni sometimes manages to befriend other animals, and forms a particularly strong bond with Gwicken, a small bouncing owl that is Martha's pet in Winds of Darkness.

Etruscan goddess, considered by the Romans to be identical to Juno.

Uni was worshipped in many Etrucan cities, often together with the sky god Tinia and the goddes Menrva (corresponding to the Roman Capitoline Triad of Jupiter, Juno and Minerva). She is considered the mother of the demigod Hercle (corresponding to Hercules).

Legend has it that the Roman priests convinced Uni to "defect" to Rome during the siege (in 392 BCE) of the Etruscan city of Veii. The Romans conquered the city and moved the statue of Uni to Rome, where she was placed in a temple of her own, on the Aventine hill.

This was written to complement, not supplement, ephealy's WU, above.